Understanding Large Negative Thermal Expansion of NdFe(CN)₆ through the Electronic Structure and Lattice Dynamics

A large negative thermal expansion (NTE) (α_v = -4.1 × 10^-5 K^-1, 100-525 K) has been discovered in NdFe(CN)₆. Here, the synchrotron X-ray diffraction and lattice dynamics calculations using the density functional theory were conducted to understand the NTE in NdFe(CN)₆. The information obtained on the bond nature of the Nd-N≡C-Fe linkage and on the atomic thermal vibrations suggests that the transverse vibrations of the -N≡C- group, in particular from N atoms, produced the NTE in NdFe(CN)₆. This is corroborated by the calculated Grüneisen parameters, which confirm the relationship between NTE and CN atomic vibrations. The results provide a helpful contribution toward the realization of new materials with negative or controllable thermal expansion.